1. Field of the Invention
The present invention relates to a position information detection system using a semiconductor device, which transmits and receives data by a radio signal.
2. Description of the Related Art
In recent years, a small semiconductor device in which a ultra-small IC chip and an antenna for wireless communication are combined with each other (hereinafter also referred to as a semiconductor device or an RF chip, and also called an RFID tag, a wireless tag, an ID tag, or an RF tag) has attracted attention. The semiconductor device can transmit and receive data without contact, for example, write data or read data, by transmission and reception of a radio signal (operating magnetic field) with use of a wireless communication device (hereinafter also referred to as a reader/writer).
As an application field of a semiconductor device which transmits and receives data by a radio signal, product management in the distribution industry is given as an example. Product management using a bar code and the like is the mainstream at present; however, since bar codes are read optically, data cannot be read in some cases when an interrupting object exists. On the other hand, when data is transmitted and received without contact with use of a wireless communication device, data of the semiconductor device is read wirelessly; thus, data can be read even when an interrupting object exists, if the interrupting object allows a wireless communication signal to pass through. Accordingly, improvement in efficiency, cost reduction, and the like of product management are expected. Further, wide range of applications including boarding passes, airplane tickets, and automatic payment of fares are expected (see Patent Document 1: Japanese Published Patent Application No. 2000-149194). A system such that people and objects are identified and managed by a small semiconductor device which transmits and receives data by wireless communication is called Radio Frequency IDentification (RFID), and has attracted attention as fundamental technology of the IT society.
Further, in recent years, research on technology in which a physical position of an RF chip is detected and a place of a product to which the RF chip is attached is specified in addition to transmission and reception of data by a radio signal has also been conducted. If the technology for detecting a physical position is developed, an effect such as shortening of time to look for things in a warehouse can be expected, for example. Further, a product to which the RF chip is attached can be traced; thus, movement of a customer having the product can be known, and a possibility of providing high value-added service can be expected, for example.
As a detection method of a physical position of an RF chip, there is a Received Signal Strength Indicator (RSSI) method, which utilizes properties that a signal intensity of a radio signal is reduced in accordance with an increase in a distance from a signal source. The RSSI method is a method where a physical position of an RF chip is identified by calculating a distance from a detected signal intensity when a position of a reader/writer is known, the reader/writer is a source of a radio signal, and the RF chip is provided with a function to detect a signal intensity. Further, the RSSI method is a method where a physical position of an RF chip is identified by calculating a distance from a detected signal intensity when a position of the reader/writer is known, the RF chip is a source of a radio signal, and a reader/writer is provided with a function to detect a signal intensity.